The Relative Value of Taxonomic Characters

Abstract

Although taxonomists may disagree on many questions, they are all likely to endorse Crow's (1926) simple statement in taxonomy some characters are of greater importance than others. One obvious reason is the fact that experience in identifying plants or dissociated plant parts has taught us to rely on good or diagnostic characters and to disregard others for this purpose. However, diagnostic characters, though indispensable in identification because of limited occurrence, often prove valueless if applied in the opposite direction toward the construction of classifications. To this end characters of wide occurrence are needed, since their value increases with their constancy in increasingly higher taxonomic categories. Thus our modern systems of classification readily reflect the results of continued observation and study on part of taxonomists, of the kinds of characters, their numbers and distribution in the different groups of the plant kingdom. At least two sets of characters are, then, constantly used according to the two basic taxonomic procedures: 1) an analytical set for the immediate purpose of identification, composed essentially of diagnostic characters of limited occurrence, and 2) a synthetic set for purposes of classification, represented by fundamental or constant characters of wide occurrence (Diels 1921, Plate 1914). Another, no less important consideration involves the nature and sources of taxonomic characters. It is safe to say that the majority of taxonomic characters is based on external morphological characters, since they still are the most easily accessible and widely used. Yet they are being supplemented more and more from other sources, i.e., the data of paleobotany, anatomy, cytology, genetics, physiology, chemistry, etc. In short, the aim and scope of phytogta. phy have been greatly expanded in keeping with the advances made by other branches of botanical science (Diels 1921, Moll 1934). Despite the fact that all characters are theoretically of equal value, practicing taxonomists have long recognized differences which result either from the nature of the characters themselves, the organic diversity of the groups of organisms to be classified, or the nature and purpose of the particular taxonomic procedures based on them. It seems, then, that the status and value of any character or group of characters are likely to vary along these lines, as will be shown below.